A passive display device such as a liquid crystal display (LCD) reflects or absorbs ambient sunlight or indoor light to display images on an LCD panel. Thus, the ambient sunlight or indoor light is required for a user to view the displayed image. However, in a case where intensity of the ambient sunlight or indoor light is not sufficient to illuminate a display panel, there is a problem in that the user cannot view the displayed image. As an alternative to such a problem, a backlight panel for backlighting a display panel is generally employed.
A backlight panel includes a light source such as an incandescent lamp, fluorescent lamp or light emitting diode (LED). Light emitted from the light source illuminates the LCD panel, and thus, images are implemented. Meanwhile, since the LED has superior reproducibility, it has been frequently used as a backlight source. In addition, since the LED is environmentally friendly, it is expected that its use will be further increased in the future.
FIG. 1 is a partial sectional view illustrating a conventional backlight panel for backlighting an LCD panel 17 using light emitting diodes (LEDs).
Referring to FIG. 1, an LED array is arranged on a printed circuit board 1. The LED array is an array in which red, green and blue LEDs 3r, 3g and 3b are arranged at a predetermined interval. The LEDs are arranged on the printed circuit board 1 in a regular order and constructed into an LED module, and a plurality of LED modules are used to backlight the LCD panel 17.
Each of the LEDs employs a lens designed for side-luminescence such that major light can be radiated almost in a side direction.
A reflection sheet 5 is positioned below light exit surfaces of the LEDs. The reflection sheet may be formed with a reflection layer 5a on the top thereof. The reflection sheet reflects light emitted from the LEDs in an upward direction.
Furthermore, a light transmission layer 7 is positioned above the LEDs. The light transmission layer is a layer through which light emitted from the LED can be transmitted. The light transmission layer is generally made of a transparent resin such as PMMA (poly methyl methacrylate). The light transmission layer 7 is provided with light shielding patterns 9 at positions corresponding to the LEDs. The light shielding patterns function to prevent light emitted upward from the LEDs from penetrating the light transmission layer and then traveling toward the LCD panel 17. The light shielding patterns may be formed through an ESR (eletroslag remelting) process.
The light transmission layer 7 and the reflection sheet 5 are spaced apart at a predetermined interval to define a first gap 6a which is an air layer. The first gap is a region where red, green and blue light emitted respectively from the LEDs 3r, 3g and 3b are mixed with one another.
A diffusion plate 11 is positioned above and spaced apart from the light transmission layer 7. The diffusion plate 11 diffuses light incident thereon to make the light uniform. The diffusion plate and the light transmission layer 7 are spaced apart from each other by a predetermined interval to define a second gap 10a which is an air layer. Thus, the light transmitted through the light transmission layer 7 is again mixed within the second gap 10a and the mixed light is then incident onto the diffusion plate 11.
A brightness enhancement film (BEF) 13 such as a prism sheet is positioned on a top surface of the diffusion plate 11. The BEF may be composed of two sheets which have upward prisms formed in longitudinal and transverse directions, respectively. In addition, a dual brightness enhancement film (DBEF) 15 such as a dual brightness enhancement film-embossed (DBEF-E) is positioned on a top surface of the BEF 13. The BEFs 13 and 15 refract light emitted at a large showing angle from the diffusion plate 11 into light with a small showing angle such that the light can be incident onto the LCD panel 17. Accordingly, the luminance of the LCD panel is increased.
Since the red, green and blue LEDs are used as light sources according to a prior art, the color reproducibility can be enhanced. However, it is necessary to mix light emitted from the LEDs in order to obtain the uniform light. Accordingly, it is necessary to prevent light emitted from the LEDs from transmitting directly through the light transmission layer 7 and traveling toward the diffusion plate 11 by using the light shielding pattern 9. Further, air layers such as the first and second gaps 6a and 10a are required. These result in an increase in thickness of a backlight panel.
Further, while light emitted from the LEDs is mixed, light loss is generated. Thus, such light loss should be compensated by increasing an amount of current supplied to the LEDs, by using a larger number of LEDs or by using the BEF 13, the DBEF 15 and the like. However, in a case where the amount of current is increased or a large number of LEDs are used, power consumption is increased. In particular, the increase in the amount of current leads to an increase of heat generated from the LEDs, and the light shielding layer may be deteriorated. In addition, the use of the BEFs causes the total thickness of the backlight panel to be further increased and the manufacturing costs of the backlight panel to be increased.
Technical Problem
An object of the present invention is to provide a backlight panel which has smaller thickness and reduced manufacturing costs as compared with a conventional backlight panel.
Another object of the present invention is to provide a backlight panel capable of enhancing its luminance as compared with a conventional backlight panel.
Technical Solution
In order to achieve the above objects of the present invention, there is provided a backlight panel employing a white light emitting diode with red and green phosphors. A backlight panel according to an aspect of the present invention comprises a light guide plate for emitting light incident from a side direction. The light guide plate emits light through a top surface thereof. The white light emitting diodes are arranged at a side of the light guide plate to emit white light into the light guide plate. Each of the white light emitting diodes includes a blue light emitting diode chip and red and green phosphors positioned over the blue light emitting diode chip. Furthermore, a diffusion plate is positioned over the top surface of the light guide plate. Therefore, the color reproducibility of a liquid crystal display can be ensured and the thickness of the backlight panel can also be reduced, by employing the white light emitting diode with three red, green and blue wavelengths.
Further, a brightness enhancement film and/or a dual brightness enhancement film may be positioned over the diffusion plate. The brightness enhancement films can enhance the luminance of the backlight panel.
A backlight panel according to another aspect of the present invention comprises a diffusion plate having a top surface and a bottom surface. White light emitting diodes are arranged below the bottom surface of the diffusion plate at a predetermined interval from the diffusion plate. Each of the white light emitting diodes includes a blue light emitting diode chip and red and green phosphors positioned over the blue light emitting diode chip. Furthermore, a reflection sheet is positioned below light exit surfaces of the white light emitting diodes and also allows light traveling in a direction opposite to the diffusion plate to be reflected toward the diffusion plate. Therefore, the color reproducibility of a liquid crystal display can be ensured and spaces or gaps where light is mixed can also be reduced, so that the thickness of the backlight panel can be reduced. Further, since the luminance can be enhanced with the use of a light source for upward emitting light, brightness enhancement films can be omitted.
In addition, a brightness enhancement film and/or a dual brightness enhancement film may be positioned over the diffusion plate to further increase the luminance of the backlight panel.
Preferably, the red phosphor of the present invention is a phosphor that is excited by blue light emitted from the blue light emitting diode chip to radiate red light. The red phosphor may be an alkali earth metal sulfide-based red phosphor expressed as general formula, Ax-aEuaGeSz, where A is at least one element selected from the group consisting of Ca and Sr; z=x+2; x is set within a range of about 2 to about 5; and a/x is set within a range of about 0.0005 to about 0.02.
Preferably, the green phosphor of the present invention is a phosphor that is excited by blue light emitted from the blue light emitting diode chip to radiate green light. The green phosphor may be a thiogallate-based green phosphor expressed as general formula, (A1-x-yEux(MI0.5MIII0.5)y)B2S4, where 0<x, y, x+y<1; A is at least one element selected from the group consisting of Ba, Sr and Ca; B is at least one element selected from the group consisting of Al, Ga and In; x is set within a range of about 0.01 to about 0.1; MI is at least one element selected from the group consisting of Li, Na and K; MIII is at least one element selected from the group consisting of Sc, Y, Lu, Gd and La; and y is set within a range of about 0.2 to about 0.8.